Since ferritic stainless steel has excellent corrosion resistance, it is used for various applications such as a kitchen or the like. In the case of stainless steel, the states of C and N present in the steel and corrosion resistance are closely connected. That is, when C and N are present in a solid solution state in the steel, Cr carbonitrides are formed during heat treatment or in a cooling process after welding to form a Cr-depleted layer in the vicinity of the Cr carbonitrides, and thereby deterioration of corrosion resistance, so-called “sensitization”, occurs in some cases. In order to suppress such sensitization, in the producing of stainless steel, countermeasures have been taken to reduce the amounts of solid-soluted C and solid-soluted N in grains by reducing the amounts of C and N as much as possible and by adding an element having higher carbonitride-forming capability (such as Nb or Ti) than that of Cr. As described above, the ferritic stainless steel is used to produce a steel sheet in which the amounts of solid-soluted C and solid-soluted N are reduced as much as possible.
On the other hand, it is known that the solid-soluted C and N remaining in the grains affect properties of the material after aging. In low-carbon steel, a Bake-Hardening (BH) phenomenon occurs in which the strength of the material is increased by performing heat treatment on the low-carbon steel at a low temperature after strain is applied to the steel in some cases. It has been considered that BH occurs due to the following. The solid-soluted C (N) remaining in grains is fixed to dislocation introduced by applying strain and then becomes an obstacle to dislocation movement. Therefore, the amount of stress required for deformation increases, that is, the strength of the material increases. It is known that there is a preferable correlation between the amount of C in the grains and the amount of stress increased by BH (bake-hardening amount, BH amount) A. A technology for controlling a BH amount by adjusting the amount of solid-soluted C has been developed (refer to NPL 1).
In regard to BH occurring in the steel type containing Cr, knowledges described in NPL 2 are known. NPL2 discloses that after the steel type containing Ti in an amount sufficient to fix C and N as carbonitrides (18Cr-0.197Ti-0.0028C-0.0054N steel) is subjected to tension of 7.5% and then to aging at 200° C. for 30 minutes, the aging index thereof is higher than 10 MPa. This result shows that even when Ti is added in an amount sufficient to fix C and N as precipitates in the stainless steel, the solid-soluted C or N is present therein.
As described above, as a countermeasure to sensitization of a ferritic stainless thin steel sheet, a method has been adopted, in which the amounts of solid-soluted C and solid-soluted N are reduced in grains by reducing the amounts of C and N as much as possible, and adding an element having higher carbonitride formation capability (such as Nb or Ti) than that of Cr. However, as disclosed in NPL 2, even when a sufficient amount of Ti is added, solid-soluted C or N remains in some cases.
Here, such a ferritic stainless thin steel sheet is subjected to cold rolling, annealing, and then skin-pass rolling in many cases. When this steel sheet is worked after being stored for a long period of time under the environment of relatively high temperature (approximately to 50° C.), a wrinkle-like shape (stretcher strain) is formed due to the occurrence of a yield point, which causes a problem in some cases. The stretcher strain is a surface defect occurring because a part of dislocation is already fixed by the solid-soluted C and solid-soluted N before processing (before strain is applied) (natural aging) to cause yield point elongation at the time of processing. The stretcher strain causes a problem in that product properties are remarkably deteriorated. In addition, since the stretcher strain spoils the outer appearance, polishing is required to remove the stretcher strain. Thus, it is important to suppress the occurrence of stretcher strain.
That is, solid-soluted C or solid-soluted N remains and stretcher strain occurs even in a high purity ferritic stainless thin steel sheet to which a carbonitride-forming element such as Ti or Nb is added. Therefore, a stringent method for storing a thin steel sheet after cold rolling is used as a countermeasure.
On the other hand, as a technique for increasing various properties by defining the details of a heat treatment condition in ferritic stainless steel to which Sn is added, techniques in PTLs 1 to 3 are known.
PTL 1 discloses a method to obtain a steel sheet satisfying both corrosion resistance and workability by revising the finish annealing conditions. PTL 2 discloses a method to obtain a steel sheet having excellent rust resistance by controlling a dew point and atmosphere at the time of finish annealing. PTL 3 discloses a method to obtain a steel sheet having excellent oxidation resistance and high temperature strength by defining conditions for hot-rolled sheet annealing and cooling after annealing.